The design of a commercially competitive fuel injection system normally involves acceptance of some characteristics which are less than optimal since the basic goals of low cost, high performance and reliability are often in direct conflict. For example, distributor-type fuel injector systems having a single centralized high pressure pump and a distributor valve for metering and timing fuel flow from the pump to each of a plurality of injection nozzles, such as disclosed in U.S. Pat. No. 3,577,765, are less expensive to construct than are other types of injection systems. However, distribution-type systems are not as reliable in operation as other types of systems due to unpredictable/uncontrollable behavior of high pressure fluids within the fluid line connecting the centralized high pressure fuel pump to the individual injector nozzles. Many of the drawbacks associated with distributor-type systems can be overcome by providing an individual cam operated unit injector at each engine cylinder location, such as illustrated in U.S. Pat. No. 3,544,008, whereby only low pressure fuel needs to be supplied to each injector, since the high pressure pressure necessary for injection can be supplied by the cam actuated pump located in the injector immediately adjacent the engine cylinder. However, unit injector systems suffer substantially higher manufacturing costs as compared with distributor type systems.
Commercially competitive fuel injector systems of the future will almost certainly need some capacity for controlling the timing of injection in response to changing engine conditions in order to achieve acceptable pollution abatement and fuel efficiency. However, high pressure distributor-type systems are probably inherently incapable of achieving the high degree of accuracy required and unit injectors which are provided with variable timing capability have invariably been highly complex and costly.
U.S. Pat. Nos. 2,997,994 and 2,863,438 provide examples of attempts to solve this dilemma by disclosing a fairly simple mechanism for achieving variable timing in unit injectors. In particular, these patents disclose the use of a collapsible hydraulic link to selectively change the effective length of the cam operated fuel injector plunger. However, the simplicity of these hydraulic timing controls is only achieved by operating the hydraulic link in either a fully expanded or fully collapsed mode. Thus, there can only be a stepped change in timing of the injection event which will not necessarily suit the broad range of conditions normally encountered during the operation of the engine. Attempts to provide for infinite variations in injection timing, even when a hydraulic link is employed, have generally involved the use of a rack mechanically connected to each injector in a manner to control the size and/or the point of collapse of the hydraulic link. Examples of such hydraulic/mechanical systems are disclosed in U.S. Pat. Nos. 3,847,510 and 4,092,264.
Independent control of fuel injection timing and quantity is critical to the achievement of highly efficient, non-polluting operation of a fuel injected internal combustion engine. However, such control must not sacrifice reliability and economy. U.S. Pat. Nos. 4,249,499 and 3,951,117 disclose fuel injectors which attempt to achieve independent control over injection timing and quantity. Each of these patents discloses an example of pressure/time unit injectors which respond to a hydraulic variable pressure signal to control injector timing. While useful for the purposes intended, the injectors disclosed in U.S. Pat. Nos. 4,249,499 and 3,951,117 do not entirely separate the timing and fuel metering functions or are too complex to achieve the desired level of low cost and reliability. For example, in U.S. Pat. No. 3,951,117 the variable timing chamber and variable metering chamber of the disclosed injector are separated only by a fixed length shuttle piston whose movement in response to change in the volume of one chamber may cause an immediate effect in the volume and/or pressure of fluid in the other chamber. The system disclosed in U.S. Pat. No. 4,249,499 discloses an infinitely variable timing system but achieves this result by provision of a fairly complex structure including a timing chamber, a pair of spring biased piston elements and external fittings located outside of the conventional injector body. Such a system could add significantly to the cost of a commercial injector.
Some attempts have been made to improve unit injector systems by having the fuel metering function performed by a centrally located fuel pump such as disclosed in U.S. Pat. No. 1,991,586 to Vincent which discloses a fuel injection system for a compression ignition engine including a plurality of cam operated pump units for injecting fuel into corresponding combustion cylinders combined with a variable stroke fuel pump (FIGS. 5 and 6) for displacing a controlled quantity of fuel into a common rail connected to all of the cam operated injectors in sequence just before each injector reaches its injection period. Although not specifically discussed, the Vincent patent states that it may in certain instances be desirable to advance or retard the beginning of injection and such may be "attained in any well known manner", page 6, left hand column, lines 57-62. The high pressure existent in the supply lines connecting the fuel pump with the cam operated units would, however, appear to subject the Vincent system to the same type of high pressure drawbacks as discussed above with respect to distributor type injection systems.
U.S. Pat. No. 3,855,982 to Brinkman discloses an injection system including reciprocating plungers for supplying metered quantities of fuel to injector nozzles associated with each cylinder of an internal combustion engine wherein the stroke length of each pump plunger may be adjusted by mechanical stops in order to control the quantity of fuel delivered to each combustion chamber. The reciprocating movement of the plungers is brought about by fluid pressures. But the Brinkman patent fails to disclose a variable timing control for the disclosed system.
In short, the prior art has failed to show how to achieve highly accurate control over injection timing in a cost competitive fuel injection system.